In recent years, consideration has been given to designing a power supply system with reduced carbon emissions, through installation of a power plant which utilizes renewable energy not accompanied by emission of greenhouse effect gases during power generation. Nevertheless, the power generation utilizing renewable energy finds difficulty in control of a power supply amount, and renders it difficult to realize stable supply of power.
In the future, if the number of such installation of power plants utilizing the renewable energy increases, a short-term demand-and-supply balance in power can be disturbed and power frequencies can deviate from the proper value. Such situations would obstruct the stable supply of power, thus can result in deterioration in power quality.
Accordingly, consideration has also been given to implementing electrical storage or electrical distribution, through installation of secondary batteries, which store surplus power from the power plant utilizing the renewable energy, on the sides of power systems and customers (e.g. buildings, factories). In this context, if electrical storage apparatuses and external devices are connected by direct current wiring, losses due to conversion to the alternating current will be avoided, and consequently, efficiency in use of the generated power will be improved.